Detecting the geographical location of wireless units

ABSTRACT

The geolocation of a wireless terminal is determined by performing a time/frequency analysis on incoming signals received at a plurality of locations, where each incoming signal includes at least one multipath component of a signal transmitted by the wireless terminal during, for example, regular communications. The time/frequency analysis involves the analysis of the frequency components of the transmitted signal at given instants in time to identify the time-of-arrival of the multipath components of the incoming signal. The time-of-arrival identified for the line-of-sight component of the incoming signal at a plurality of locations can then be processed to determine the geolocation of the wireless terminal.

RELATED APPLICATIONS

This invention is related to the inventions disclosed in the applicant'sapplications: Ser. No. 08/984,779, now U.S. Pat. No. 6,175,811, entitled“Method For Frequency Environment Modeling and Characterization,” filedon Dec. 4, 1997; Ser. No. 08/984,728, now U.S. Pat. No. 6,272,350entitled “Method For Improved Line Of Sight Signal Detection UsingTime/Frequency Analysis,” filed on Dec. 4, 1997; and Ser. No.08/984,780, now U.S. Pat. No. 6,259,894 entitled “Method For ImprovedLine-Of-Sight Signal Detection Using RF Model Parameters, filed on Dec.4, 1997.

FIELD OF THE INVENTION

The present invention relates to the field of radio communications and,more particularly, to a method and apparatus for locating wirelessterminals.

BACKGROUND OF THE INVENTION

There are a number of different types of communications systems thatrequire information regarding the location of an object or element inorder to operate efficiently.

For example, some wireless communications systems require informationregarding the location of a receiving unit in order to efficiently routesignals from a transmitting unit to the receiving unit, and vice versa.

In cellular communications systems, for example, the cellular network(i.e. the network of base stations) requires information regarding theidentity of the cell in which a wireless terminal is located in order toefficiently route signals to and from the wireless terminal. Once thecell is identified, the cellular network can send a signal to thewireless terminal through a base station that provides coverage to theidentified cell. This enables the cellular network to deliver the signalto the wireless terminal without having to transmit the signal throughevery base station. As a result, the cellular network is able to avoidconsuming bandwidth in those cells in which the wireless terminal is notresident, and thus increase the overall efficiency of the cellularsystem.

Although present-day cellular networks identify the cell in which thewireless terminal is located, they do not identify the exact geolocationof the wireless terminal in that cell. The term geolocation as usedherein refers to the point in two or three-dimensional space defined bya set of coordinates (e.g. longitude and latitude) and/or defined by avector (i.e. distance and direction) from a known point in space. Thislack of geolocation information greatly decreases the efficiency of thecellular system when the wireless terminal moves from cell to cell. Inaddition, the lack of geolocation information reduces the number ofservices that a given cellular system can provide (e.g. roadsideassistance, fleet management, etc.).

To illustrate, cellular systems in North America presently use aso-called system-wide paging approach to identify the cell in which awireless terminal is located. Pursuant to that approach, the wirelessterminal periodically transmits identification information, referred toas registration, to the cellular network. Depending on the location ofthe wireless terminal, the registration signal can be received by anynumber of base stations, each base station covering a specific cell. Thecellular network identifies the cell in which the wireless terminal islocated by comparing the strength of the registration signal received ateach base station. The base station that receives the strongestregistration signal is identified as the cell in which the wirelessterminal is presently located. Once the “present” cell is identified,the cellular system can then communicate with the wireless terminalthrough the base station that covers that cell.

If the wireless terminal moves out of the present cell, the cellularsystem must send a page to the wireless terminal through a plurality ofcells and wait for the wireless terminal to send another registration.Once the new registration is received, the cellular system can thenidentify the new cell in which the wireless terminal is located bycomparing the signal strengths, as described above. As a result, thecellular network must consume additional bandwidth to periodicallyidentify the cell in which the wireless terminal is located.

In addition, if the user of the wireless terminal makes a call for help(e.g. because the user is in distress), the wireless network can notidentify the geolocation from which the wireless user made the call. Asa result, unless the user can accurately identify his or her location,the wireless network can provide only limited help in the dispatch ofaid to the user. Thus, present-day cellular systems in North America,for example, are not very useful to other service providers, such asroadside assistance and medical emergency care.

One solution to this problem is to equip the wireless terminal with theability to identify its own geolocation. For example, the wirelessterminal can be equipped with a Global Positioning System (GPS) receiverthat receives GPS signals and uses those signals to determine thegeolocation of the wireless terminal. Once determined, the geolocationinformation can be periodically sent to the cellular network. This wouldenable the cellular network to periodically identify the geolocation ofthe wireless terminal without having to consume additional bandwidthsending pages. In addition, this solution would enable the cellularnetwork to track the movement of the wireless terminal from cell tocell, and thus predict and/or anticipate the time to hand-offcommunications from one cell to the next. As a result, this solutionwould enable the cellular network to increase the overall efficiency ofcommunications, and to provide additional information to serviceproviders such as roadside assistance providers (e.g. to located a userin distress).

The just described solution, however, is disadvantageous because theadditional hardware needed to equip the wireless terminal with theability to identify its own geolocation would increase the price, sizeand weight of the wireless terminal to unappealing proportions. Indeed,there is an ongoing effort by those skilled in the art to reduce theprice, size and weight of present-day wireless terminals (e.g. cellphones).

Techniques are known, outside the cellular communications arena, foridentifying the geolocation of a transmitting unit. For example, in somesatellite communications systems, the geolocation of a transmitting unitis identified by determining the times in which the line-of-sightcomponents of a signal transmitted from the transmitting unit reached arespective receiver locations. The line-of-sight component of theso-called incoming signal at each receiver location is that component ofthe signal that propagated directly from the wireless terminal to thelocation at which the signal was received (i.e. the receiver location)without scattering or reflecting off structures in the environment. Todetermine the time-of-arrival of the line-of-sight component of theincoming signal, such satellite geolocation systems assume that thefirst-arriving component of the incoming signal is the line-of-sightcomponent. Then, based on the time-of-arrival of the first-arrivingcomponent of the incoming signal at, for example, three receiverlocations, the geolocation of the wireless terminal is calculated.

Such conventional geolocation systems, however, are hindered by theirfailure to consider the problems associated with scattering in the RFenvironment. Scattering refers to the phenomenon wherein signalstraveling in an RF environment reflect off structures in theenvironment, and thus scatter in various different directions or pathsin the RF environment. Specifically, scattering may cause a signal totravel more than one path between two points (e.g. a wireless terminaland a receiver location) in an RF environment. This so-called multipathphenomenon may cause an incoming signal at a receiver location to becomposed of a plurality of so-called multipath components (i.e. repeatedversions of the transmitted signal). Thus, depending on the propensityof the RF environment to scatter a signal, referred to herein as thescattering hostility, the incoming signal at the receiver location maybe composed of a number of such multipath components.

Scattering and/or multipathing may cause the first-arriving component ofthe incoming signal to arrive at the receiver location very close intime to the time-of-arrival of the next-arriving multipath component ofthe incoming signal. A conventional geolocation system may not be ableto distinguish between the two components, causing it to mistakenlydetermine that the time of arrival of the first-arriving component is atsome intermediate time between the time of arrival of the first-arrivingcomponent and the time-of-arrival of the next-arriving multipathcomponent, i.e., a later time than the first-arriving component actuallyarrived. This so-called time-shift of the identified time-of-arrival maycause the geolocation system to determine an incorrect geolocation ofthe wireless terminal.

In addition, depending on the scattering hostility of the RFenvironment, the line-of-sight path between a wireless terminal (i.e.the transmitting unit) and a base station (i.e. the receiver location)may be blocked (e.g. by a building). When this happens, thefirst-arriving component of the incoming signal (at the receiverlocation) will not be the actual line-of-sight component, but rathersome later-arriving component. Again, this would cause the conventionalgeolocation systems to incorrectly assess the time-of-arrival of theline-of-sight component as being received at a later point in time, i.e.time-shifted, thereby causing the geolocation system to inaccuratelycalculate the geolocation of the wireless terminal.

As a result, it can be understood that there are at least two differentmechanisms wherein scattering and/or multipathing may cause a so calledtime-shift of the determined time-of-arrival of the line-of-sightcomponent of the incoming signal, and thus be problematic to theaccuracy of identifying the geolocation of a wireless terminal byconventional geolocation systems.

SUMMARY OF THE INVENTION

The above-described problems are ameliorated in accordance with theprinciples of the invention. In particular, the geolocation of awireless terminal is determined by a performing a time/frequencyanalysis of a signal transmitted by the wireless terminal during, forexample, regular communications. This approach allows the terminal to beidentified with more accuracy than is achieved by attempting to identifythe time-of-arrival of the first-arriving multipath component of theincoming signal; without having to add substantial hardware to thewireless terminal, and without having to consume additional bandwidth onthe wireless network with which the wireless terminal communicates.

The term time/frequency analysis as used herein refers to an analysis ofthe frequency components (i.e. the frequency make-up) of a signal atgiven instants in time. For example, one form of time/frequency analysisaccording to the present invention is to compare the frequency make-upof the received signal to the frequency make-up of the transmittedsignal. Those points in time in which the frequency make-up of thereceived signal matches the frequency make-up of the transmitted signalare the instants in time at which a particular component of thetransmitted signal is received.

In accordance with a feature of the invention, the time/frequencyanalysis is used to identify the time-of-arrival of the line-of-sightcomponent of the incoming signal. By performing such a time/frequencyanalysis on the line-of-sight components of the incoming signalsreceived at, for example, three locations, the geolocation of thewireless terminal can be identified with better accuracy than in theprior art because the identified time-of-arrival at each location ismore accurate than that identified in the prior art.

In preferred embodiments, the accuracy of the identified time-of-arrivalof the line-of-sight component of the incoming signal at each receiverlocation is improved by an amount based on the value of at least oneparameter of an RF model that characterizes the scattering hostility ofthe RF environment in which the respective incoming signal traveled. Byadjusting the time-of-arrival in this way, the above-discussedtime-shift due to scattering is compensated for, and thus moreaccurately reflects the time at which the line-of-sight component of theincoming signal would have arrived if the RF environment werescatter-free. This increased accuracy of the identified time-of-arrivalof the line-of-sight component of the incoming signal will cause anincrease in the accuracy of the determination of the geolocation of thewireless terminal.

In particular embodiments, the time/frequency analysis mayadvantageously be carried-out using the signal processing disclosed inco-pending application Ser. No. 08/984,728, entitled “Method ForImproved Line Of Sight Signal Detection Using Time/Frequency Analysis,”filed on even date herewith.

In addition, in particular embodiments, an identified time-of-arrival ofthe line-ofsight component of the incoming signal can be adjusted usingthe methods disclosed in co-pending application, Ser. No. 08/984,780,entitled “Method For Improved Line-Of-Sight Signal Detection Using RFModel Parameters,” filed of even date herewith.

Also, in particular embodiments, the parameters of the RF model used toadjust the identified times-of arrival can be advantageously carried-outusing the methods disclosed in co-pending application Ser. No.08/984,779, entitled “Method For Frequency Environment Modeling AndCharacterization,” filed of even date herewith.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is block diagram of an illustrative embodiment of a method fordetermining the geolocation of a wireless terminal in accordance withthe principles of the present invention.

FIG. 2 is a top-view of the paths of the line-of-sight and multipathcomponents of a signal traveling from a wireless terminal to a pluralityof receiver locations in an RF environment.

FIG. 3 is a graphical view of the various components of an illustrativeincoming signal received at a receiver location shown in FIG. 2.

FIG. 4 is a graphical view of the various components of an illustrativeincoming signal received at another receiver location shown in FIG. 2.

FIG. 5 is a graphical view of a wavelet representation of a sawtoothsignal helpful in explaining how, in preferred embodiments, theabove-described time shift is determined.

FIG. 6 is a block diagram of a set of linear filters that can be used todecompose an incoming signal into a set of frequency components in orderto identify the time-of-arrival of a component of the incoming signal.

FIG. 7 is a block diagram of an illustrative embodiment of a method forusing wavelet analysis as a form of time/frequency analysis to identifythe line-of-sight component of an incoming signal in accordance with theprinciples of the present invention.

FIG. 8 is block diagram of an illustrative embodiment of a method fordetermining whether a wavelet representation of an incoming signalmatches a wavelet representation of a transmitted signal.

FIG. 9 is a block diagram of an illustrative embodiment of an apparatusfor adjusting the identified time-of-arrival of an incoming signal.

FIG. 10 is a block diagram of an illustrative embodiment of a method fordetermining a basis parameter used to adjust an identifiedtime-of-arrival of an incoming signal.

FIG. 11 is a block diagram of an illustrative embodiment of a method forforming an RF model of an RF environment.

FIG. 12 is a block diagram of an illustrative embodiment of a method fordetermining the geolocation of a wireless terminal based on adjustedtimes-of-arrival in accordance with the principles of the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE OF THE INVENTION

Referring now to FIG. 1 there is shown a method 10 for determining thegeolocation of a wireless terminal in accordance with the principles ofthe present invention. As shown, method 10 begins with step 11 wherein asignal transmitted by the wireless terminal is received at a pluralityof receiver locations. The signal received at each receiver location isreferred to herein as the incoming signal. A time/frequency analysis isperformed, in accordance with the principles of the present invention,on each incoming signal, at step 12, to identify the time-of-arrival ofthe first-arriving multipath component of the respective incoming signalat each receiver location. The identified time-of-arrival of thefirst-arriving multipath component at each location is adjusted, at step13, by an amount based on the value of at least one parameter of an RFmodel that characterizes the scattering hostility of the RF environmentin which the respective incoming signal traveled. The resultant“adjusted” time-of-arrival of the first-arriving component of eachincoming signal more accurately reflects the time the line-of-sightcomponent of the incoming signal would have reached the respectivereceiver locations if the RF environment were scatter-free. The various“adjusted” times-of-arrival are then processed, at step 14, to determinethe geolocation of the wireless terminal.

In order to more fully explain the operation of method 10, the followingtopics will be addressed: (1) the incoming signal; (2) the line-of-sightand multipath components of the incoming signal; (3) the “time-shift” ofthe line-of-sight component of the incoming signal due to scattering;(4) time/frequency analysis of the incoming signal in accordance withthe principles of the present invention; (5) identifying thetime-of-arrival of the line-of-sight component of the incoming signal inaccordance with the principles of the present invention; (6) using an RFmodel to reduce a “time-shift” of the identified time-of-arrival of theline-of-sight component of the incoming signal, due to scattering; (7)determining parameters that form the RF model used to reduce the timeshift; and (8) using the time-of-arrival of the line-of-sight componentof the incoming signal received at a plurality of locations to determinethe geolocation of the wireless terminal, in accordance with theprinciples of the present invention.

The Incoming Signal

The incoming signal can be any signal transmitted by the wirelessterminal including those signals transmitted during wirelesscommunications. The term regular communications as used herein refers tothe communications the wireless terminal regularly performs when inoperation, for example, communications involving a call between the userof a cell phone and another party.

Line-Of-Sight and Multipath Components

As described above, the incoming signal may be composed of any number ofmultipath components of the transmitted signal, depending on thescattering hostility in the RF environment in which a signal travels. Anillustration of the physical process by which various multipathcomponents arrive at different receiver locations in an RF environment20 is shown in FIG. 2. As shown, a wireless terminal 21, buildings 24-27and receiver locations 22 and 23 are all located at different positionsin RF environment 20. Depending on the position of wireless terminal 21with respect to buildings 24-27 and receiver locations 22 and 23, asignal transmitted from wireless terminal 21 travels a plurality ofpaths 28-32 therefrom. Specifically, a signal transmitted from wirelessterminal 21 travels to receiver location 22 along line-of-sightmultipath, or line-of sight path, 30 and multipath 29, and to receiverlocation 23 along multipaths 28 and 32.

Time-Shift Due to Scattering

The incoming signal received at receiver location 22 has both aline-of-sight component (i.e. the component that traveled alongline-of-sight path 30) and a multipath component (i.e. the componentthat traveled along multipath 29). Since the multipath component travelsa longer distance then the line-of-sight component, the time-of-arrivalof the multipath component is later than the time-of-arrival of theline-of-sight component.

Referring now to FIG. 3 there is shown a graphical view of the line-ofsight component S₃₀ and the multipath component S₃₂ of an incomingsignal received at receiver location 22, where the incoming signal isshown in FIG. 3 as signal 36. As shown, the line-of sight component S₃₀of signal 36 has a time-of-arrival 34 and the multipath component S₃₂ ofsignal 36 has a time-of-arrival 35. As stated above, if time-of-arrival34 and time-of-arrival 35 are very close in time, conventionalgeolocation devices that use matched filters to determine thetime-of-arrival of the line-of-sight component may inaccuratelydetermine that the line-of-sight component S₃₀ of incoming signal 36arrived at time 31, which is Δt later than it actually arrived. Whenthis happens, the identified time-of-arrival (i.e. time 31) of theline-of-sight component S₃₀ is said to be “time-shifted” by the timeshift Δt due to scattering and/or multipathing in the RF environment.

The time shift Δt due to such close-arriving multipaths can be reducedby performing a time/frequency analysis on the incoming signal inaccordance with the present invention, as described below.

Referring now back to FIG. 2, the incoming signal received at receiverlocation 23 has two multipath components, i.e., the components thattraveled along multipaths 28 and 32. However, it has no line-of-sightcomponent because line-of-sight path 31 is blocked by building 27. Sincesuch multipath components travel a farther distance in RF environment 20than would the line-of-sight component if it had not been blocked bybuilding 27, the time-of-arrival of each multipath component istherefore, by definition, later than the time at which the line-of-sightcomponent would have arrived at receiver location 23.

Referring now to FIG. 4, there is shown a graphical view of an incomingsignal 33 received at receiver location 23. As shown, incoming signal 33includes a multipath component S38 that traveled along path 28, amultipath component S39 that traveled along path 32, and an expectedline-of-sight component S37 that never reaches location 23 because it isblocked by building 27. The multipath components s38 and s39 are shownto have times-of-arrival 38 and 39 respectively, and expected line-ofsight component s37 is shown to have an expected time-of-arrival 37,which is the time-of-arrival that line-of sight component s37 would havehad to receiver location 23 had it never been blocked by building 27.Since a conventional geolocation system, as described above, wouldassume that the line-of-sight component of incoming signal S33 is theiving component, such a geolocation system would incorrectly assume thatthe time-of-arrival of the line-of-sight component of incoming signal 33is time 38, as opposed to time 37. When this happens, the geolocationsystem would incorrectly assume that the time-of-arrival of theline-of-sight component s37 was a time shift Δt, i.e., time differencebetween time 37 and time 38, later than it should have arrived. Asdescribed above, such a time shift would reduce the accuracy of thegeolocation system in determining the geolocation of wireless terminal21.

The time shift Δt, due to such a blocked line-of-sight path, can bereduced by adjusting the identified time-of-arrival of thefirst-arriving component of the incoming signal by an amount based on aparameter that characterizes the scattering hostility of the RFenvironment in which the incoming signal traveled.

Time/Frequency Analysis

The term time/frequency analysis as used herein refers to an analysis ofthe frequency components of a signal (e.g. the magnitude of the waveformat each component frequency of the signal) at given instants in time.For example, one form of time/frequency analysis, in accordance with theprinciples of the present invention, is to compare the frequencycomponents of the incoming signal to the frequency components of thetransmitted signal at given instants in time. (Although the exact natureof the transmitted signal is not typically known, its frequencycomponents can indeed be known to a great extent since they are afunction of the carrier frequency and the modulation used.) In such atime/frequency analysis, those instants in time in which the frequencycomponents of the incoming signal match the frequency components of thetransmitted signal are the instants in time at which a multipathcomponent is received.

One illustrative method for performing a time/frequency analysis on theincoming signal, in accordance with the principles of the presentinvention, is to perform a so-called wavelet analysis on the incomingsignal. Wavelet analysis involves the act of breaking down a signal intoa set of simpler elements, called wavelets. The wavelets are basicallylocalized waveforms that last for only a few cycles. Thus, according towavelet analysis, a wavelet representation of a signal is the set ofwavelets that can be superposed to form the waveform of the signal.

Wavelet analysis can be explained by analogy to Fourier analysis. AFourier transform represents a signal as a superposition of sinusoidswith different frequencies, and the Fourier coefficients represent thecontribution of the sinusoid at these frequencies. Similarly, a wavelettransform represents a signal as a sum of wavelets with differentwidths, called dilations, and amplitudes, called scalings, and thewavelet coefficients provide a measure of the contributions of eachwavelet at these dilations and scalings.

For example, referring now to FIG. 5 there is shown a sawtooth signal 52that is represented by a family of wavelets, or wavelet representation,51. As shown, wavelet representation 51 includes 16 different wavelets,each having a different dilation and scaling. For example, wavelet 53has a dilation 54 and a scaling 55, and wavelet 56 has a dilation 57 anda scaling 58. Wavelet representation 51 is referred to as the wavelettransform of sawtooth signal 52, and sawtooth signal 52 is referred toas the inverse transform of the wavelet representation 51.

Both the wavelet transform and the inverse transform are arrived ataccording to known algorithms. For example, one illustrative algorithmused to compute both the wavelet transform and the inverse transform ofa waveform is the fast pyramid algorithm described by A. Bruce, D.Donoho and H. Y. Goo, in “Wavelet Analysis,” IEEE Spectrum, October1996, and incorporated herein by reference. The fast pyramid algorithmprovides a “forward algorithm” that serves to compute the wavelettransform, and a “backward algorithm” that serves to compute the inversetransform. The forward algorithm uses a series of linear filters todecompose a signal into a set of filtered components. It is thewaveforms of these filtered components that form the waveletrepresentation of the signal. The reverse algorithm uses a set of linearfilters to combine the wavelets comprising the wavelet representation toform the signal.

Referring now to FIG. 6 there is shown a block diagram of anillustrative set of linear filters 600, used to decompose a signal 601into a set of filtered components 615-622. As shown, the set of linearfilters 600 has a first-line filter 602 which is connected tosecond-line filters 603 and 604. Second line filter 603 is connected tothird-line filters 605 and 606, and second-line filter 604 is connectedto third-line filters 607 and 608.

In operation, signal 601 is input into first-line filter 602 whichdecomposes signal 601 into a high-frequency filtered component 609 and alow-frequency filtered component 610. Second-line filter 503 decomposeshigh-frequency component 609 into high frequency filtered components 611and 612, and second-line filter 604 decomposes low frequency filteredcomponent 610 into low-frequency filtered components 613 and 614.Third-line filter 605 decomposes high frequency filtered component 611into high frequency filtered components 615 and 616. Third-line filter606 decomposes high-frequency filtered component 612 into high frequencyfiltered components 617 and 618. Third-line filter 607 decomposeslow-frequency filtered component 613 into low-frequency filteredcomponents 619 and 620. Third-line filter 608 decomposes low-frequencyfiltered component 614 into low-frequency filtered components 621 and622.

The waveforms of the set of filtered components 615-622 that result byinputting signal 601 into the set of linear filters 600 is shown. Thewaveforms of frequency components 615-622 are a set of waveforms thatcan be superposed to form signal 601. As a result, it is the waveformsof frequency components 615-622 that form a wavelet representation ofsignal 601. Alternatively, a different set of linear filters may bechosen to decompose signal 601 into a different set of frequencycomponents that provide a different set of waveforms, or wavelets. Thus,it can be understood that different sets of linear filters can decomposesignal 601 into different wavelet representations.

Identifying the Time-Of-Arrival

The above-described time/frequency analysis can be used, in accordancewith the principles of the present invention, to identify thetime-of-arrival of the line-of-sight component of an incoming signal.For example, wavelet analysis can be used as a time/frequency analysisto identify the time-of-arrival of the line-of sight component of anincoming signal. Referring now to FIG. 7 there is shown one illustrativeembodiment of a method 70 for using wavelet analysis for identifying thetime-of-arrival of the line-of-sight component of an incoming signal, inaccordance with the principles of the present invention.

As shown, method 70 begins at step 71 wherein the incoming signal ispassed through a set a set of linear filters to obtain a waveletrepresentation of the incoming signal. The wavelet representation of theincoming signal identified at each instant in time is then compared, atstep 72, to a wavelet representation of a transmitted signal. As statedabove, although the exact nature of the transmitted signal is not known,the waveform of its frequency components can indeed be known to a greatextent since they are a function of the carrier frequency and the typeof modulation used. Thus, since the wavelet representation of a signalis dependent on the waveform of its frequency components, the waveletrepresentation of the transmitted signal can be estimated with greataccuracy.

The instants in time wherein the wavelet representation of the incomingsignal is substantially similar to, or matches, the waveletrepresentation of the transmitted signal (called the expected waveletrepresentation) are identified, at step 73, as the times-of-arrival ofthe multipath components of the incoming signal. Since the line-of-sightcomponent of the incoming signal travels the shortest distance, thefirst-arriving, or earliest, time-of-arrival is identified, at step 74,as the time-of-arrival of the line-of-sight component of the incomingsignal. As stated above, determining the time-of-arrival of theline-of-sight component using such a time/frequency analysis, reducesthe inaccuracies associated with using a matched filter. Thus, method 70increases the accuracy of the identified time-of-arrival of theline-of-sight component of the incoming signal over the prior artmethods that utilize such a so-called matched filter method.

An illustrative embodiment of a method 80 for determining whether thewavelet representation of the incoming signal matches the expectedwavelet representation of the transmitted signal is shown in FIG. 8. As,shown, method 80 begins at step 81 wherein the dilations and scalings ofthe wavelets which represent the incoming signal are compared to thedilations and scalings of the “corresponding wavelets” which representthe transmitted signal. The term “corresponding wavelets” as used hereinrefers to the wavelet of the incoming signal and the wavelet of thetransmitted signal that represent the waveform of the frequency band ofits respective signal. At step 82, the corresponding wavelets that havedilations and scalings that are within some tolerance of each other areidentified, and labeled as matching wavelets. The number of matchingwavelets is then compared to a threshold number, at step 83. If thenumber of matching wavelets is equal to or greater than the thresholdnumber then, at step 84, the wavelet representation of the incomingsignal is said to match the wavelet representation of the transmittedsignal. If, however, the number of matching wavelets is less than thethreshold number then, at step 85, the wavelet representation of theincoming signal is said to not match the wavelet representation of thetransmitted signal. As stated above, each instant at which a match isidentified can therefor be identified as a time-of-arrival of amultipath component of the incoming signal

Using an RF Model to Adjust an Identified Time-Of-Arrival

As described above and shown in FIGS. 2 and 4, the identifiedtime-of-arrival of the line-of-sight component of the incoming signalcan be “time-shifted” when the line-of-sight component is blocked (i.e.prevented from arriving at the receiver location).

The present inventor has found that the amount of time-shift due to sucha blocked line-of-sight path directly depends on the scatteringhostility of the RF environment in which the incoming signal traveled.In particular, the present inventor has found that the amount oftime-shift due to such a blocked line-of-sight path can be reduced byadjusting the identified time-of arrival of the first-arriving componentof the incoming signal by an amount based on a parameter thatcharacterizes the scattering hostility of the RF environment in whichthe incoming signal traveled.

Referring now to FIG. 9 there is shown one illustrative embodiment of adevice 90 for adjusting the identified time-of-arrival of theline-of-sight component of an incoming signal. As shown, device 90 hasan RF model 91 connected to a processor 92. RF model 91 has a set ofparameters, each parameter characterizing the scattering hostility of agiven region of an RF environment. Processor 92 has inputs 93 and 94,and output 95.

In operation, processor 92 obtains, at input 93, the time-of-arrivalidentified for the first-arriving component of an incoming signal thattraveled through a given region of an RF environment. Processor 92obtains, at input 94, a parameter that characterizes the scatteringhostility in the given region of the RF environment, from RF model 91.The given region through which the incoming signal traveled isdetermined by identifying the direction from which the incoming signaltraveled and/or the strength of the incoming signal. The methods bywhich such signal direction and signal strength are determined are wellknown in the art. Based on the obtained parameter, processor 92determines the amount of time-shift of the obtained time-of-arrival thatis due to scattering and/or multipath in the RF environment. Then, basedon the time-shift information, processor 92 computes and outputs fromoutput 95 an adjusted time-of-arrival that more accurately reflects thetime at which the line-of-sight component would have arrived in ascatter-free environment.

As stated above, each parameter is a measure of the amount ofmultipathing, and the amount of “time-shift” that a given signal wouldincur in a given region of the RF environment. As a result, eachparameter of RF model 91 indicates the relative amount of “time-shift”that would occur if a signal were to travel in the respective region. Itcan therefore be understood that a parameter of RF model 91 thatcharacterizes a given region as having a greater scattering hostilitythan another region, necessarily indicates that the amount of“time-shift” that would occur if the incoming signal were to travel inthat given region is greater than the amount of time shift that wouldoccur in the other region.

Processor 92 can determine the amount of such a time shift that wouldoccur in a given region by comparing the value of a parameter thatcharacterizes the scattering hostility of the given region to the valueof a basis parameter. The term basis parameter as used herein refers toa parameter that characterizes the scattering hostility in a givenregion wherein the amount of time-shift that results when a signaltravels in that given region is known. Thus, a basis parameter is aparameter that has a known associated time-shift.

Referring now to FIG. 10, there is shown an illustrative embodiment of amethod 100 for determining a basis parameter. As shown, method 100begins at step 101 wherein a signal is transmitted in a regioncharacterized by the basis parameter. The time at which the signal isreceived at a known distance from the wireless terminal is determined atstep 102. The difference between the actual time-of-flight (the time ittook for the first-arriving component of the signal to actually travelthe given distance) and the expected time-of-flight (i.e. the time thesignal would have traveled the given distance along the line-of-sightpath) is determined at step 103. The calculated difference is therebythe known time-shift associated with the basis parameter.

Referring now back to FIG. 9, processor 92 determines the amount of timeshift of the time-of-arrival obtained at input 93 by comparing theparameter obtained at input 94 to a so-called basis parameter, andadjusting the value of the known time-shift associated with the basisparameter by an amount based on the difference between the value of thebasis parameter and the value of the given parameter. For example, theknown time-shift associated with the basis parameter can be adjusted indirect proportion to the difference between the value of the basisparameter and the value of the given parameter. The resultant adjustedtime shift is the time shift of the identified time-of arrival of thefirst arriving component of the incoming signal, due to such aline-of-sight path, as described above. Processor 92 can then adjust theidentified time of arrival, obtained at input 93, by the time-shift, andoutput the adjusted time-of-arrival through output 95.

Determining the Parameters

The term parameter as used herein refers to any parameter or so-calleddimension that is capable of defining or describing a chaotic process orsystem in terms of a measure of some aspect of that system. One type ofparameter or dimension that can be used to define such a chaotic processis a fractal dimension. Fractal dimensions are described by A. P.Pentland in “Fractal-Based Description of Natural Scenes,” IEEETransactions on Pattern Analysis and Machine Intelligence, Vol. PAMI-6,No. 6, November 1984, and incorporated herein by reference.

As noted above, a fractal dimension is a parameter that defines achaotic system by characterizing the system in terms of a measure ofso-called self-similarity. For example, a fractal dimension has beenused to define the shape of a mountainous landscape by characterizingthe amount of self-similarity that exists in the shape of the landscape.The amount of self-similarity in the shape of the landscape is thenumber of times a particular shape is repeated in the shape of thelandscape itself The particular shape is the largest shape found in theactual landscape that can be used to define or re-create each piece ofthe actual landscape. As a result, the fractal dimension determined forany given landscape provides a measure of the size of the particularrepeated shape with respect to the size of the landscape itself, andthus a characterization of the landscape itself.

Just like a fractal dimension can be used as parameter to characterizethe a chaotic system such as a mountainous landscape, so can a fractaldimension be used as a parameter to characterize a chaotic system suchas an RF environment. In particular, a fractal dimension can be used tocharacterize the scattering hostility of an RF environment by providinga measure of the number of times a similar shape (i.e. the variousmultipath components of the incoming signal) is repeated in the waveformof the incoming signal that traveled in the RF environment. By providinga measure of the number of multipath components of the incoming signal,the fractal dimension actually provides a measure or characterization ofthe scattering hostility of the RF environment. Thus, a set of suchfractal dimensions forms an RF model of the RF environment.

Referring now to FIG. 11 there is shown an illustrative method 110 forforming such an RF model of an RF environment. As shown, method 110begins at step 111 wherein an RF signal having a given waveform istransmitted at a known time from a given region of an RF environment.The RF signal, after traveling a given distance in the given region ofthe RF environment is received, step 112. The received signal, orso-called incoming signal, is analyzed, step 113, to determine aparameter that characterizes the scattering hostility of that region ofthe RF environment. Then, at decision step 114, method 110 checkswhether a parameter has been determined for each region of the RFenvironment. If such a parameter has been determined for each region,method 110 ends, otherwise steps 111-113 are repeated until such timethat each region has a parameter determined therefor.

It should be understood that no region of the RF environment is limitedto being characterized by one such parameter. Rather each region can becharacterized, for example, by a plurality of parameters, or a singleparameter that is an average of a plurality of parameters. In additionan RF environment is not limited to a specific number of regions. The RFenvironment, for example, can be a single region or one hundred regions.

Moreover, each parameter of a given RF model is not limited tocharacterizing a region of any particular size. For example, a given RFmodel can be compose of a set of parameters wherein each parametercharacterizes the scattering hostility in a region having the same sizeand shape as every other region. Or, for example, each parameter cancharacterize a region having an arbitrary size and shape. Or, forexample, each parameter characterizes a region having size and shapebased on some criterion such as the physical profile (i.e. rural, urban,suburban, etc.) of the region.

In addition, a set of such parameters that characterize any given regioncan be determined as a function of time. That is, each determinedparameter may be a time-varying function of the scattering hostility ofa given region of the RF environment.

Advantageously, a set of such parameters can be used as an RF model ofthe RF environment to aid in the design of a wireless communicationsystem. For example, since each parameter defines the amount ofmultipath that a given signal would incur if the signal were to travelin a region of the RF environment, each parameter can be used to predictthe amount a signal would multipath if the signal were to propagate inthat respective region. Based on the prediction, a system designer couldestimate the amount the waveform of the RF signal would change shape asa result of traveling in the given region, and thus could determinewhether a given receiver would be capable of detecting and/orrecognizing the transmitted signal after traveling in the RFenvironment. Such information, as described above, may be critical intesting the design of a wireless system before incurring the cost ofbuilding the system itself

In addition, a set of such parameters can advantageously be used toadjust the identified time of arrival of the line-of-sight component ofthe incoming signals received at a plurality of locations in an RFenvironment, and thus provide a geolocation system with more-accuratetime-of-arrival information for determining the geolocation of awireless terminal operating in the RF environment.

Determining the Geolocation of a Wireless Terminal

Referring now to FIG. 12, there is shown a method 120 for determiningthe geolocation of a wireless terminal, according to the principles ofthe present invention. As shown, method 120 begins at step 121 whereinthe adjusted time-of-arrival of the line-of-sight component isidentified for the incoming signal received at a plurality of receiverlocations. Then, step 122, the various times-of-arrival are processed todetermine the distance of the wireless terminal from at least threereceiver locations. From this distance information, the geolocation ofthe wireless terminal is identified, step 123.

The processing performed to determine the geolocation of a wirelessterminal, based on the time-of-arrival of the line-of-sight component ofthe incoming signal received at the at least three receiver locations,is well-known in the art. For example, one illustrative method for usingsuch times-of-arrival information to determining the geolocation of avehicle is disclosed by J. Brooks Chadwick and J. L. Bricker in “AVehicle Location Solution Approach,” IEEE Position Location andNavigation Symposium, 1990, and incorporated herein by reference.

It should be noted that using an adjusted time-of-arrival of theline-of-sight component of the various incoming signals, as computedabove, increases the accuracy of the just-described processing fordetermining the geolocation of the wireless terminal. This is due to thedirect dependence of the accuracy of the geolocation calculation on theaccuracy of time-of-arrival of the line-of-sight component of theincoming signal. Advantageously, determining the geolocation of awireless unit as described above, does not require the consumption ofadditional bandwidth, or the increased in cost associated with addinghardware to the wireless terminal, as in some of the prior artsolutions.

While the invention has been particularly shown and described withreference to various embodiments, it will be recognized by those skilledin the art that modifications and changes may be made to the presentinvention without departing from the spirit and scope thereof. As aresult, the invention in its broader aspects is not limited to specificdetails shown and described herein. Various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims.

I claim:
 1. A method of determining geolocation for a wireless terminal,comprising the steps of: receiving incoming signals from the wirelessterminal at a plurality of locations including at least one multipathcomponent; performing, at each location, a time/frequency analysis on afirst-arriving component of a respective incoming signal transmitted toeach location by the wireless terminal, so as to identify thetime-of-arrival of the first-arriving component of the incoming signalat each location; determining an adjusted time-of-arrival at eachlocation for the identified first-arriving component, by an amount thatis based on a fractal dimension that characterizes the scatteringhostility of a given region in an RF environment through which theincoming signal has traveled from the wireless terminal to theparticular location, said fractal dimension being compared against abasis parameter that has a known associated time-shift, the differencebeing used to determine the adjusted time-of-arrival for saidfirst-arriving component at each location, the adjusted time-of-arrivaldetermination thereby compensating for any time-shift between the firstarriving component and a next-arriving component due to scattering; anddetermining the geolocation of said wireless terminal based on saidadjusted time-of-arrival determination.
 2. The method of claim 1 whereinsaid step of determining the geolocation of the wireless terminalincludes the step of determining the distances between said wirelessterminal and at least three of said plurality of locations, based onsaid determined times-of-arrival of the first-arriving component of eachof said incoming signals.
 3. The method of claim 1, wherein thegeolocation of said wireless terminal is determined from adjustedfirst-arriving components for at least three locations.
 4. The method ofclaim 1 wherein said step of performing the time/frequency analysisincludes performing wavelet analysis on each of said incoming signals.5. An apparatus for determining geolocation for a wireless terminal,comprising: means for receiving incoming signals at a plurality oflocations including at least one multipath component; means forperforming, at each location, a time/frequency analysis on afirst-arriving component of a respective incoming signal transmitted toeach location by the wireless terminal, so as to identify thetime-of-arrival of the first-arriving component of the incoming signalat each location; means for determining an adjusted time-of-arrival ateach location for the identified first-arriving component, by an amountthat is based on a fractal dimension that characterizes the scatteringhostility of a given region in an RF environment through which theincoming signal has traveled from the wireless terminal to theparticular location, said fractal dimension being compared against abasis parameter that has a known associated time-shift, the differencebeing used to determine the adjusted time-of-arrival for saidfirst-arriving component at each location, the adjusted time-of-arrivaldetermination thereby compensating for any time-shift between the firstarriving component and a next-arriving component due to scattering; andmeans for determining the geolocation of said wireless terminal based onsaid adjusted time-of-arrival determination.
 6. The apparatus of claim 5wherein said means for determining the geolocation of the wirelessterminal includes means for determining the distances between saidwireless terminal and at least three of said plurality of locations,based on said determined times-of-arrival of the first-arrivingcomponent of each of said incoming signals.
 7. The apparatus of claim 5wherein said means for determining the geolocation of said wirelessterminal determines the geolocation of said wireless terminal fromadjusted first-arriving components for at least three locations.
 8. Theapparatus of claim 5 wherein said means for performing thetime/frequency analysis includes performing wavelet analysis on each ofsaid incoming signals.